In the manufacture of substrates, for example, printed circuit (“PC”) boards, it is frequently necessary to apply small amounts of viscous materials, i.e. those with a viscosity greater than fifty centipoise. Such materials include, by way of example and not by limitation, general purpose adhesives, solder paste, solder flux, solder mask, grease, oil, encapsulants, potting compounds, epoxies, die attach pastes, silicones, RTV and cyanoacrylates.
In the quest for ever increasing miniaturization of circuitry, a fabrication process known as flip chip technology has developed, which has multiple processes that require viscous fluid dispensing. For example, a semiconductor die or flip chip is first attached to a PC board via solder balls or pads, and in this process, a viscous solder flux is applied between the flip chip and the PC board. Next, a viscous liquid epoxy is allowed to flow and completely cover the underside of the chip. This underfill operation requires that a precise amount of the liquid epoxy be deposited in a more or less continuous manner along at least one side edge of the semiconductor chip. The liquid epoxy flows under the chip as a result of capillary action due to the small gap between the underside of the chip and the upper surface of the PC board. Once the underfill operation is complete, it is desirable that enough liquid epoxy be deposited to encapsulate all of the electrical interconnections, so that a fillet is formed along the side edges of the chip. A properly formed fillet ensures that enough epoxy has been deposited to provide maximum mechanical strength of the bond between the chip and the PC board. Thus, underfilling with the epoxy serves first, as a mechanical bond to help reduce stress and limit strain on the interconnecting solder pads during thermal cycling and/or mechanical loading and second, protects the solder pads from moisture and other environmental effects. It is critical to the quality of the underfilling process that the exact amount of epoxy is deposited at exactly the right location. Too little epoxy can result in corrosion and excessive thermal stresses. Too much epoxy can flow beyond the underside of the chip and interfere with other semiconductor devices and interconnections.
In another application, a chip is bonded to a PC board. In this application, a pattern of adhesive is deposited on the PC board; and the chip is placed over the adhesive with a downward pressure. The adhesive pattern is designed so that the adhesive flows evenly between the bottom of the chip and the PC board and does not flow out from beneath the chip. Again, in this application, it is important that the precise amount of adhesive be deposited at exact locations on the PC board.
The PC board is often being carried by a conveyor past a viscous material dispenser that is mounted for two axes of motion above the PC board. The moving dispenser is capable of depositing dots of viscous material at desired locations on the PC board. There are several variables that are often controlled in order to provide a high quality viscous material dispensing process. First, the weight or size of each of the dots may be controlled. Known viscous material dispensers have closed loop controls that are designed to hold the dot size constant during the material dispensing process. It is known to control the dispensed weight or dot size by varying the supply pressure of the viscous material, the on-time of a dispensing valve within the dispenser and the stroke of an impact hammer in a dispensing valve. Each of those control loops may have advantages and disadvantages depending on the design of a particular dispenser and the viscous material being dispensed thereby. However, those techniques often require additional components and mechanical structure, thereby introducing additional cost and reliability issues. Further, the responsiveness of those techniques is proving less satisfactory as the rate at which dots are dispensed increases. Therefore, there is a continuing need to provide better and simpler closed loop controls for controlling dot size or weight.
A second important variable that may be controlled in the dispensing process is the total amount or volume of viscous material to be dispensed in a particular cycle. Often the designer of a chip specifies the total amount or volume of viscous material, for example, epoxy in underfilling, or adhesive in bonding, that is to be used in order to provide a desired underfilling or bonding process. For a given dot size and dispenser velocity, it is known to program a dispenser control, so that the dispenser dispenses a proper number of dots in order to dispense a specified amount of the viscous material in a desired line or pattern at the desired location on the PC board. Such a system is reasonably effective in a world in which the parameters that effect the dispensing of the viscous material remain constant. However, such parameters are constantly changing, albeit, often only slightly over the short term; but the cumulative effect of such changes can result in a detectable change in the volume of fluid being dispensed by the dispenser. Therefore, there is a need for a control system that can detect changes in dispensed weight and automatically adjust the dispenser velocity, so that the desired total volume of viscous material is uniformly dispensed over a whole dispensing cycle.
A third important variable relates to the timing of dispensing dots of viscous material on-the-fly. When dispensed on-the-fly, the dots of viscous material fly horizontally through the air prior to landing on the PC board. In order to accurately locate the dots on the PC board, it is known to perform a calibration cycle in which a time based compensation value is determined and used to pre-trigger the dispenser. Again, there is a need to continue to improve the process by which an on-the-fly dispenser can dispense dots of viscous material, so that they are more accurately located on the PC board.
Therefore, there is a need for an improved computer controlled viscous fluid dispensing system that addresses the needs described above.